With the advent of DNA-based molecular technologies, microbial ecologists now have the tools to test whether general ecological patterns apply to microorganisms. In this study, we selected 11 high-mountain lakes from Sierra Nevada (Spain) to test the predictions of island-biogeography theory in relation to ecosystem size and isolation, and to assess the influence of other factors (i.e., ecosystem productivity, resource richness, and biotic interactions) on bacterial community structure. Bacterial operational taxonomic units (OTUs), generated by denaturing-gradient gel electrophoresis of polymerase chain-reactionamplified 16S rRNA genes, were used as a surrogate of predominant ''biodiversity units.'' OTU composition among lakes was heterogeneous, and the number of site-specific OTUs was near 50%. Lake remoteness did not affect the number of bacterial OTUs although the spatial distribution of the lakes significantly influenced bacterial composition. Lakes that were closer together had more similar bacterial fingerprints. We found a consistent positive association between bacterial OTU richness and lake area. The slope of this relationship (0.161 Ϯ 0.026, including literature data) was similar to slopes obtained for organisms with high dispersion rates.

Bottle incubations, during which the activity and growth of prokaryotes is monitored during several days, are frequently carried out to study functional aspects of marine prokaryotic assemblages. These experiments will relate directly to in situ activities if all populations grow harmonically during the incubation. We tested whether this was the case by analyzing the composition of bacterial assemblages at the beginning and at the end of the incubation by denaturing gradient gel electrophoresis. Five experiments were done in different Antarctic regions. Bacterial assemblages north and south of the Polar Front were very different. In all cases, the final assemblages were very different from the initial ones, and these changes were often accompanied by a significant decrease of diversity indices. Our experiments included treatments with different temperature and organic matter amendments. Whereas the increase in temperature tested had a minor effect on prokaryotic growth rate and specific composition, the addition of organic matter strongly stimulated growth rate and selected a particular bacterial assemblage in some experiments but not in others. A significant component of bacterial assemblages from waters south of the Polar Front appeared to be Polaribacter franzmannii, a gas vacuolated bacterium of the CytophagaߚFlavobacteriumߚBacteroides group that was originally isolated from Antarctic sea ice. This phylotype was enriched and dominated in almost all final assemblages. Our results indicate that longߚterm bottle incubations mostly measure the activity of a few opportunistic bacteria and not that of the original assemblage. This should be taken into account if data obtained in these experiments are used for balancing whole ecosystem carbon budgets and to derive biogeochemical conclusions.

The surface film of the hydrosphere covers more than 70% of the world's surface. The sea surface microlayer (SML) or "skin" of the ocean is a sink for natural and anthropogenic material originating from the atmosphere and the water column. Organisms living in this SML are called "neuston." Our knowledge of the biology of the SML is still in its infancy. Research of the sea surface microlayer requires the use of appropriate sampling techniques and strategies, and the question of what is the most suitable device has not yet been answered. In the present study, we have compared the efficiency of the Harvey glass plate (GP) and the Garrett metal screen (MS) to analyze a wide range of microbiological parameters in SML samples collected at two coastal stations in the NW Mediterranean Sea. Two types of membranes (Teflon and polycarbonate) were also used to collect bacterioneuston. The MS was the most appropriate technique for most biological parameters providing higher enrichment factors as compared to the GP and, therefore, the highest enrichment factors compared with underlying waters (UW). Control experiments with UW demonstrated that the enrichment reported for the MS was not biased by any selectivity of the sampler itself. Therefore, we recommend the use of the MS when the aim is to compare different biological parameters. In contrast, there is clear evidence that hydrophobic and hydrophilic membranes have an important drawback and should not be used for quantification purposes. AcknowledgmentsThis work was supported by the European Commission (Research Directorate General-Environment Program-Marine Ecosystems) through the AIRWIN project "Structure and role of biological communities involved in the transport and transformation of persistent pollutants at the marine AIR-Water Interface" (contract EVK3-CT2000-00030). The AIRWIN project is part of the EC IMPACTS cluster. We thank Frédérique François-Carcaillet for helpful comments on statistical analysis and Danielle Boissé for language improvements. We are also grateful to the two anonymous reviewers for valuable comments on a previous version of the manuscript.